Task: Remove std::map inheritance, clean up interface

- Make the Task object's interface more explicit by removing the
  std::map inheritance.
- Using this more explicit interface, remove unneeded ctors in order to
  allow the compiler to "Do The Right Thing"(tm).
  This leads to a performance improvement of 12% in the "add"
  performance test, and 7% for "import".

test/t.t.cpp

@@ -189,7 +189,7 @@ TODO Task::decode
   test.is (task.get ("three"), "four", "three=four");
 
   // Task::set
-  task.clear ();
+  task.data.clear ();
   task.set ("name", "value");
   test.is (task.composeF4 (), "[name:\"value\"]", "Task::set");
 
@@ -213,7 +213,7 @@ TODO Task::decode
   test.is (task.composeF4 (), "[name:\"value\"]", "Task::remove");
 
   // Task::all
-  test.is (task.size (), (size_t)1, "Task::all size");
+  test.is (task.data.size (), (size_t)1, "Task::all size");
 
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